This invention relates to analog circuitry and, more particularly, to a novel analog voltage isolation circuit, which can be implemented with relatively few components, and at low cost.
The coupling of an analog voltage from one location in an apparatus, such as a power supply, to another location within that same apparatus, can be complicated when the common, or reference, potentials at the different locations are not the same. It is well-known that certain apparatus, such as floating amplifiers or power supplies, higher-voltage power supplies and the like, can often contain sections thereof which are referenced to a common potential xe2x80x98floatingxe2x80x99 tens, hundreds or even thousands of volts above or below a chassis potential, which itself may even be floating with respect to local earth ground potential. It is thus often necessary to isolate one analog voltage signal from others, especially with respect to different common potentials against which the analog signals are each referenced. In the past, isolated analog voltage measurements have been accomplished by use of isolation amplifiers (in discrete or integrated circuit form), pulse transformer circuitry, linear optically-coupled isolation circuitry (opto-isolators) and the like. Isolation amplifiers generally operate well, but are very expensive; pulse transformer circuits are somewhat less expensive as long as high accuracy is not required, but more precise measurements require complex circuitry and generate higher cost. Linear optically-coupled isolation circuits require special high-linearity, multiple-output opto-couplers, which are also relatively expensive and require a significant amount of support circuitry. Such opto-couplers also require either self-calibration circuitry or individual manual calibration of the circuit, to account for absolute gain differences between opto-coupler outputs.
A high-accuracy, low-cost novel analog voltage isolation circuit is thus desirable.
In accordance with the invention, a novel analog voltage isolation circuit includes a first subcircuit for generating a pulse-width-modulated (PWM) electrical parameter having at least one of a frequency and a duty cycle variably responsive to an input signal value, and a second subcircuit, isolated from the first subcircuit, for converting the PWM parameter to an output signal substantially equal to the input signal.
In a presently preferred embodiment, the first subcircuit includes an error amplifier-integrator having an error-output voltage for comparison to a selected level referenced to the same common potential as the input signal being measured. The comparator output varies the current flowing through the series-connected light-emitting diodes of first and second opto-isolators. The output of the first opto-isolator, with respect to the input ground potential, generates a first current fedback to the error amplifier-integrator, to establish both the frequency and duty-cycle of a rectangular-wave PWM signal periodically varying proportional to the magnitude of the input voltage. In the second subcircuit, the same periodically-varying current flows through the second opto-isolator input circuit, to generate an output voltage which varies substantially linearly with variance of the circuit input voltage signal, but is referenced to another common potential, isolated from the input common potential.